The invention thus finds applications in the automotive field, and in particular in the field of road vehicle lighting and signaling, and also in the field of video surveillance.
There already exist devices that enable the presence of fog to be detected. Such devices are used in particular for automatically causing lights and lighting devices to be switched on and for enabling drivers to adapt the speed of their vehicles as a function of the visibility of the road scene situated in front of their vehicles.
One such device for detecting the presence of fog makes use of an anticollision device of the light detection and ranging (LIDAR) type that serves to evaluate transmission through the atmosphere in order to deduce the presence of fog. Unfortunately, LIDARs are devices that are very expensive; it is therefore difficult to envisage installing them systematically on transport vehicles.
Another device, as proposed in document U.S. Pat. No. 6,853,453, makes use of targets arranged in the scene under study in order to detect the presence of a disturbing element. That method requires targets to be placed in the scene, which is very constricting, and impossible for mobile applications.
Another device, proposed by document JP 11-278182, is based on identifying and characterizing the light halo that appears around the taillights of a vehicle that appears in the scene being studied. The drawback of that system is that it does not operate when there is no vehicle light that appears in the scene.
Another device, proposed by document US 2008/0007429, makes use of the backscattering of the light from the headlights of the vehicle in which the device is arranged. Nevertheless, that system is not sufficiently reliable, in particular because it is based on very local analysis of the image and it is found to be ineffective for detecting the presence of an element disturbing the visibility in certain environments, in particular in environments that are fairly well lit.
It can thus be understood that a relatively large number of devices have been proposed for detecting fog at night. Nevertheless, none of those devices is genuinely satisfactory. Each of those devices is effective only in a limited operating range, and is found to be incapable of detecting fog effectively under other atmospheric conditions.